1. Field of the Invention
The present invention relates to a novel electrically conducting ceramic that can be desirably used as an electrode material in an electrochemical apparatus operated at high-temperature conditions in addition to being used for the isolation of gases and electrolysis in a vapor phase.
The invention further relates to a solid oxide fuel cell and, particularly, to an improvement of an air electrode formed on one surface of a solid electrolyte.
2. Description of the Prior Art
Ceramic materials have heretofore been used in a variety of applications owing to their multiple functions. Among them, a ceramic having electrically conducting property is finding applications in various fields where it is not allowed to used metals to substitute for the metals that are good electrically conducting materials.
In a solid oxide fuel cell, for instance, electrodes are deposited on both sides of the electrolyte. Such a fuel cell makes use of an electrically conducting ceramic of the type of oxide as a cathode electrode since the environment in which it is used is as severe as 800.degree. C. or higher.
The solid oxide fuel cells that are known so far can be divided into two types, i.e., those of the tubular type and those of the planar type. The fuel cell of the planar type has an advantage of power generation of a high power density per a unit volume but involves problems in regard to incomplete gas sealing and non-uniform temperature distribution in the cell when it is to be put into practical use. On the other hand, the fuel cell of the tubular type has a low power density but presents such features that the cell has a large mechanical strength and the temperature is maintained uniform in the cell. Study has been positively forwarded concerning the solid oxide fuel cells of these two types in order to utilize their respective features.
In the fuel cell of the tubular type, a unit cell is obtained as shown in FIG. 3 by forming a porous air electrode 2 composed of an LaMnO.sub.3 -type material by the slurry-dip method on the surface of a support tube 1 composed of a CaO-stabilized ZrO.sub.2 having an open porosity of about 40%, forming on the surface thereof a Y.sub.2 O.sub.3 stabilized ZrO.sub.2 electrolyte 3 by the vapor-phase synthesizing method (EVD) or the spraying method, and forming a fuel electrode 4 composed of porous Ni-zirconia cermet surface of the solid electrolyte. In a module of fuel cell, a plurality of the thus constituted single cells are connected together via an interconnector 5 composed of an LaCrO.sub.3 -type material or the like material. The electric power is generated by flowing the air (oxygen) into the support tube 1 and flowing the fuel (hydrogen) into the outer portion at a temperature of 1000.degree. to 1050.degree. C.
In order to fabricate the fuel cell through a simplified process in recent years, it has been attempted to use the LaMnO.sub.3 -type material which is the air electrode directly as a porous support tube. As the support tube material which also functions as the air electrode, use is made of the LaMnO.sub.3 solid solution in which La is substituted by 10-20 atomic % of Ca or 10-15 atomic% of Sr.
In a single cell of the fuel cell of the planar type which uses the same materials as those of the tubular type fuel cell as shown in FIG. 4, on the other hand, a porous air electrode 9 is provided on one side of a solid electrolyte 8 and a porous fuel electrode 10 is provided on the other side thereof. The single cells are connected together by a separator 11 composed of the LaCrO.sub.3 -type material in which is solid-dissolved MgO or CaO which is a dense material. The electric power is generated by feeding the air (oxygen) toward the side of the air electrode and feeding the fuel (hydrogen) toward the side of the fuel electrode at a temperature of 1000.degree. to 1050.degree. C.
In the above-mentioned LaMnO.sub.3 perovskite-type composite oxide, however, the electrical conductivity is about 150 to 200.OMEGA..sup.-1. cm.sup.-1 in the open air at 1000.degree. C. In can be expected to increase the formation of holes by substituting La in an increased amount with a divalent metal element. When La is substituted by more than about 30%, however, there is formed a crystal phase which is different from the perovskite type, and the electrical conductivity is not improved.
Furthermore, according to the report of Mori et al. in Electrical Central Laboratory Report (W90002) dated Feb. 7, 1990, LaMnO.sub.3 -type perovskite compounds have three type crystalline structures according to temperatures, and have a problem of lacking stability at high temperatures because the thermal expansion behaviors of these crystalline phases are different. The report tells that stability at high temperatures is improved by substituting a part of La with a moderate amount of Sr or Ca. But in a system wherein a part of La is substituted with Sr or Ca, the crystalline structure changes. Thus, a substantial improvement is not attained. On the other hand, Japanese Laid-Open Patent Publication No. 303565/1992 proposes that annealing treatment is carried out at a temperature of 800 to 1400.degree. C. in order to solve a problem that La (Sr, Ca) MnO.sub.3 is unstable at high temperatures. Its effect is not sufficient.
Moreover, in a cell of the structure in which the CaO-stabilized ZrO.sub.2 is used as the support tube and is equipped with the LaMnO.sub.3 material as the air electrode in which is solid-dissolved SrO, and in a tubular fuel cell of the structure which uses the air electrode directly as the support tube, when a power generating system using this material is operated for a long period of time, the cell is deformed or the output decreases.